JP-2006-258279-A discloses a vehicle 100 in which front wheels are driven by a main drive source such as an engine (not shown) and rear wheels 102 are driven by an auxiliary drive source such as an electric motor 103 via a power transmission mechanism 104.
This power transmission mechanism 104 includes a speed reduction mechanism 105 into which, power from the electric motor 103 is inputted and a differential gear 106 which distributes power outputted from the speed reduction mechanism 105 to left and right rear wheels 102, 102. The speed reduction mechanism 105 includes a speed reduction gear train which is made up of a first gear 105a which is fixed to an output shaft of the electric motor 103, a second gear 105b which meshes with the first gear 105a, and a third gear 105e which meshes with an input gear 106a of the differential gear 106. A hydraulic clutch 107 is provided between the second gear 1056 and the third gear 105c. When the hydraulic clutch 107 is applied, the second gear 105b and the third gear 105c are coupled together, whereby power from the electric motor 103 can be transmitted to the rear wheels 102 via the power transmission mechanism 104. When the hydraulic clutch 107 is released, the coupling between the second gear 105b and the third gear 105c is released, whereby the transmission of power from the electric motor 103 to the rear wheels 102 is cut off.
In the power transmission mechanism 104 described in JP-2006-258279-A, in order to transmit the power of the electric motor 103 to the rear wheels 102, the hydraulic clutch 107 needs to be applied strongly and rigidly so as to be held in a large torque capacity state. Therefore, for example, a delay in response may be caused when the fluid temperature is low.
In order to solve the delay in response, for example, a one-way clutch may be further provided in the power transmission mechanism 104 so that the hydraulic clutch 107 is applied when the vehicle moves forwards. However, in this case, the one-way clutch is also applied when the reverse-direction rotation is inputted from the rear wheels to the electric motor 13. That is when the vehicle is reversed by driving the front wheels with the engine (main drive source) while the electric motor (auxiliary drive source) 103 is stopped, the one-way clutch is also applied. As a result of this unintentional application of the one-way clutch, when the vehicle reverses, the electric motor 103 is idly rotated in accordance with the rotation of the rear wheels, and the efficiency of vehicle energy may be deteriorated.